248 HISTORY OF BOTANY 



periments, Ricca also shows that after a length of tissue 

 had been exposed to a temperature of 150° for thirty hours 

 the leaflets closed at night and reopened next day, so that 

 the stimulus must be transmitted by non-living elements, 

 and intercellular protoplasmic fibrillae are excluded. 

 Macdougal also showed that stimuli were transmitted 

 through tissues killed by heat. Ricca's view is that 

 there are certain substances in the cell sap which he 

 considers homologous with the animal " hormones," and 

 which, after they have entered the xylem vessels, may 

 be transmitted to and stimulate the motile organ. Boysen, 

 Jensen, and De Paal have also suggested that certain 

 tropisms may be explained by the transmission of specific 

 substances. If this be so we must revise completely 

 all our ideas as to stimulus and response in plants, but 

 the data available do not warrant us as yet in coming 

 to any general conclusions on the subject. 



Ecology 



In 1895 Warming of Copenhagen pubUshed a book 

 which he termed Plantesamfund or Plant Communities, 

 which presented for the first time a detailed exposition 

 of what has come to be known as " Ecology " — a subject 

 which discusses " how plants or plant communities 

 adjust their forms and modes of behaviour to actually 

 operating factors, such as the amounts of available 

 water, heat, light, nutriment, and so forth." Warming's 

 pioneer work made its appearance, in a much improved 

 and extended form and in Enghsh dress, in 1909. In 

 1898, soon after Warming's original treatise was published, 

 another work of great importance appeared. Plant 

 Geography, upon a Physiological Basis, from the pen of 

 the talented young botanist A. F. W. Schimper, who 

 died in 1901 at the early age of 45. I think it may be 

 safely said that to these two works we owe the foundations 

 of our knowledge of ecology, a subject which bulks so 



